In an electric motor including a plurality of permanent magnets disposed along an inner circumferential surface of a cylindrical yoke for supplying field flux to a motor armature, normally, the motor casing is formed by closing the axial ends of the cylindrical yoke with a pair of end plates, and joining the end plates to the cylindrical yoke by passing a pair of set bolts axially through the two end plates. Each of such set bolts is required to be fitted into a hole provided in one of the end plates, and after being passed through the interior of the yoke, is required to be properly fitted and threaded in an associated threaded hole provided in the other end plate. Because the set bolts are relatively long, and the threaded hole is not visible to the assembling personnel, there has been some difficulty in aligning the free threaded end of the set bolt with the associated threaded hole. In particular, this difficulty has prevented full automatization of the assembly work.
In such an electric motor, the permanent magnets are made of material having magnetically desirable properties, but are often highly brittle. Therefore, it has been proposed, for instance in Japanese patent publication No. 5-59661, to fit a cylindrical magnet cover inside an inner circumferential surface defined by the permanent magnets to reinforce the permanent magnets against chipping and prevent any fragments of the permanent magnets from dislodging from the permanent magnets.
Noting the above-mentioned difficulty in assembling the motor, and the need for the provision of a magnet cover, the inventors have discovered that by providing a flange to the magnet cover and forming bolt guide holes in the flange, it is possible to aid the aligning of the bolts passed through one of the end plates with the associated threaded holes formed in the other end plate.